Liquid crystal display

ABSTRACT

According to one embodiment, a liquid crystal display includes a first substrate including a main pixel electrode, a second substrate including a main common electrode extending substantially in parallel to the main pixel electrode on both sides of the main pixel electrode, and a liquid crystal layer including liquid crystal molecules held between the first substrate and the second substrate. A horizontal inter-electrode distance in a first direction between the main pixel electrode and the main common electrode is in a range of 11 μm or more and 13 μm or less, and a dielectric constant anisotropy of the liquid crystal layer is 10 or more.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2012-062387, filed Mar. 19, 2012, theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to the liquid crystaldisplay.

BACKGROUND

In recent years, flat-panel displays have been vigorously developed. Byvirtue of such advantageous features as light weight, small thicknessand low power consumption, special attention has been paid to liquidcrystal displays among others. In particular, in active matrix liquidcrystal displays in which switching elements are incorporated inrespective pixels, attention is paid to the configuration which makesuse of a lateral electric field (including a fringe electric field),such as an IPS (In-Plane Switching) mode or an FFS (Fringe FieldSwitching) mode. Such a liquid crystal display of the lateral electricfield mode includes pixel electrodes and a counter-electrode, which areformed on an array substrate, and liquid crystal molecules are switchedby a lateral electric field which is substantially parallel to a majorsurface of the array substrate.

On the other hand, there has been proposed a technique wherein a lateralelectric field or an oblique electric field is produced between a pixelelectrode formed on an array substrate and a counter-electrode formed ona counter-substrate, thereby switching liquid crystal molecules.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view which schematically illustrates a structure and anequivalent circuit of a liquid crystal display according to anembodiment.

FIG. 2 is a plan view which schematically shows a structure example of apixel at a time when a liquid crystal display panel shown in FIG. 1 isviewed from a counter-substrate side.

FIG. 3 is a schematic cross-sectional view, taken along line A-A in FIG.2, showing a cross-sectional structure of the liquid crystal displaypanel shown in FIG. 2.

FIG. 4 shows an example of the relationship between a refractive indexanisotropy and a dielectric constant anisotropy, with respect to liquidcrystal materials which are applied to the liquid crystal display of theembodiment and a liquid crystal display of a comparative example.

DETAILED DESCRIPTION

In general, according to one embodiment, a liquid crystal displaycomprises a first substrate including a main pixel electrode; a secondsubstrate including a main common electrode extending substantially inparallel to the main pixel electrode on both sides of the main pixelelectrode; and a liquid crystal layer including liquid crystal moleculesheld between the first substrate and the second substrate. A horizontalinter-electrode distance in a first direction between the main pixelelectrode and the main common electrode is in a range of 11 μm or moreand 13 μm or less, and a dielectric constant anisotropy of the liquidcrystal layer is 10 or more.

Embodiments will now be described in detail with reference to theaccompanying drawings. In the drawings, structural elements having thesame or similar functions are denoted by like reference numerals, and anoverlapping description is omitted.

FIG. 1 is a view which schematically shows a structure and an equivalentcircuit of a liquid crystal display according to an embodiment.

Specifically, the liquid crystal display includes an active-matrix-typeliquid crystal display panel LPN. The liquid crystal display panel LPNincludes an array substrate AR which is a first substrate, acounter-substrate CT which is a second substrate that is disposed to beopposed to the array substrate AR, and a liquid crystal layer LQ whichis disposed between the array substrate AR and the counter-substrate CT.The liquid crystal display panel LPN includes an active area ACT whichdisplays an image. The active area ACT is composed of a plurality ofpixels PX which are arrayed in a matrix of m×n (m and n are positiveintegers).

The liquid crystal display panel LPN includes, in the active area ACT,an n-number of gate lines G (G1 to Gn), an n-number of storagecapacitance lines C (C1 to Cn), and an m-number of source lines S (S1 toSm). The gate lines G and storage capacitance lines C extendsubstantially linearly, for example, in a first direction X. The gatelines G and storage capacitance lines C are alternately arranged inparallel along a second direction Y crossing the first direction X. Inthis example, the first direction X and the second direction Y aresubstantially perpendicular to each other. The source lines S cross thegate lines G and storage capacitance lines C. The source lines S extendsubstantially linearly along the second direction Y. It is not alwaysnecessary that each of the gate lines G, storage capacitance lines C andsource lines S extend linearly, and a part thereof may be bent.

Each of the gate lines G is led out of the active area ACT and isconnected to a gate driver GD. Each of the source lines S is led out ofthe active area ACT and is connected to a source driver SD. At leastparts of the gate driver GD and source driver SD are formed on, forexample, the array substrate AR, and are connected to a driving IC chip2 which incorporates a controller.

Each of the pixels PX includes a switching element SW, a pixel electrodePE and a common electrode CE. A storage capacitance CS is formed, forexample, between the storage capacitance line C and the pixel electrodePE. The storage capacitance line C is electrically connected to avoltage application module VCS to which a storage capacitance voltage isapplied.

In the present embodiment, the liquid crystal display panel LPN isconfigured such that the pixel electrodes PE are formed on the arraysubstrate AR, and at least a part of the common electrode CE is formedon the counter-substrate CT, and liquid crystal molecules of the liquidcrystal layer LQ are switched by mainly using an electric field which isproduced between the pixel electrodes PE and the common electrode CE.The electric field, which is produced between the pixel electrodes PEand the common electrode CE, is an oblique electric field which isslightly inclined to an X-Y plane which is defined by the firstdirection X and second direction Y, or to a substrate major surface (ora lateral electric field which is substantially parallel to thesubstrate major surface).

The switching element SW is composed of, for example, an n-channelthin-film transistor (TFT). The switching element SW is electricallyconnected to the gate line G and source line S. The switching element SWmay be of a top gate type or a bottom gate type. In addition, asemiconductor layer of the switching element SW is formed of, forexample, polysilicon, but it may be formed of amorphous silicon.

The pixel electrodes PE are disposed in the respective pixels PX, andare electrically connected to the switching elements SW. The commonelectrode CE is disposed common to the pixel electrodes PE of pluralpixels PX via the liquid crystal layer LQ. The pixel electrode PE andcommon electrode CE are formed of a light-transmissive, electricallyconductive material such as indium tin oxide (ITO) or indium zinc oxide(IZO), but may be formed of other metallic material such as aluminum.

The array substrate AR includes a power supply module VS for applying avoltage to the common electrode CE. The power supply module VS isformed, for example, on the outside of the active area ACT. The commonelectrode CE is led out to the outside of the active area ACT, and iselectrically connected to the power supply module VS via an electricallyconductive member (not shown).

FIG. 2 is a plan view which schematically shows a structure example ofone pixel PX at a time when the liquid crystal display panel LPN shownin FIG. 1 is viewed from the counter-substrate side. FIG. 2 is a planview in an X-Y plane.

The pixel PX shown in FIG. 2 has a rectangular shape having a lesslength in the first direction X than in the second direction Y, asindicated by a broken line. A storage capacitance line 01 and a storagecapacitance line C2 extend in the first direction X. A gate line G1 isdisposed between the storage capacitance line C1 and storage capacitanceline C2 which neighbor each other, and extends in the first direction X.A source line S1 and a source line S2 extend in the second direction Y.A pixel electrode PE is disposed between the source line S1 and sourceline S2 which neighbor each other. In addition, the pixel electrode PEis disposed between the storage capacitance line C1 and storagecapacitance line C2.

In the example illustrated, in the pixel PX, the source line S1 isdisposed at a left side end portion, and the source line S2 is disposedat a right side end portion. Strictly speaking, the source line S1 isdisposed to extend over a boundary between the pixel PX and a pixelneighboring on the left side, and the source line S2 is disposed toextend over a boundary between the pixel PX and a pixel neighboring onthe right side. In addition, in the pixel PX, the storage capacitanceline C1 is disposed at an upper side end portion, and the storagecapacitance line C2 is disposed at a lower side end portion. Strictlyspeaking, the storage capacitance line C1 is disposed to extend over aboundary between the pixel PX and a pixel neighboring on the upper side,and the storage capacitance line C2 is disposed to extend over aboundary between the pixel PX and a pixel neighboring on the lower side.The gate line G1 is disposed at a substantially central part of thepixel.

A switching element SW in the illustrated example is electricallyconnected to the gate line G1 and source line S1. The switching elementSW is provided at an intersection between the gate line G1 and sourceline S1. A drain line of the switching element SW is made to extendalong the source line S1 and storage capacitance line C1, and iselectrically connected to the pixel electrode PE via a contact hole (notshown) which is formed at an area overlapping the storage capacitanceline C1. The switching element SW is provided at an area overlapping thesource line S1 and storage capacitance line C1, and hardly extrudes fromthe area overlapping the source line S1 and storage capacitance line C1,thus suppressing a decrease in area of an aperture portion whichcontributes to display.

The pixel electrode PE includes a main pixel electrode PA and a contactportion PC which are electrically connected to each other. The mainpixel electrode PA extends linearly in the second direction Y from thecontact portion PC to the vicinity of the lower side end portion of thepixel PX. In this embodiment, the width in the first direction X of themain pixel electrode PA is about 5 μm. The main pixel electrode PA isformed in a strip shape having a substantially uniform width in thefirst direction X. The main pixel electrode PA is disposed at asubstantially middle position between the source line S1 and source lineS2, that is, at the center of the pixel PX. The distance in the firstdirection X between the source line S1 and main pixel electrode PA issubstantially equal to the distance in the first direction X between thesource line S2 and main pixel electrode PA. The contact portion PC islocated at an area overlapping the storage capacitance line C1 and iselectrically connected to the switching element SW via a contact hole(not shown). The contact portion PC is formed to have a greater widththan the main pixel electrode PA.

The common electrode CE includes main common electrodes CA. The maincommon electrodes CA linearly extend, in the X-Y plane, in the seconddirection Y substantially in parallel to the main pixel electrode PA, onboth sides of the main pixel electrode PA. Alternatively, the maincommon electrodes CA are opposed to the respective source lines S andextend substantially in parallel to the main pixel electrode PA. In thepresent embodiment, the width in the first direction X of the maincommon electrode CA is about 5 μm. The main common electrode CA isformed in a strip shape having a substantially uniform width in thefirst direction X.

In the example illustrated, two main common electrodes CA are arrangedin parallel in the first direction X, and are located at left and rightend portions of the pixel PX, respectively. In the description below, inorder to distinguish these main common electrodes CA, the main commonelectrode located on the left side in the Figure is referred to as“CAL”, and the main common electrode located on the right side in theFigure is referred to as “CAR”. The main common electrode CAL is opposedto the source line S1, and the main common electrode CAR is opposed tothe source line S2. The main common electrode CAL and main commonelectrode CAR are electrically connected to each other within the activearea or outside the active area.

In the pixel PX, the main common electrode CAL is disposed at the leftside end portion, and the main common electrode CAR is disposed at theright side end portion. Strictly speaking, the main common electrode CALis disposed to extend over a boundary between the pixel PX and a pixelneighboring on the left side, and the main common electrode CAR isdisposed to extend over a boundary between the pixel PX and a pixelneighboring on the right side.

Paying attention to the positional relationship between the main pixelelectrode PA and the main common electrodes CA, the main pixel electrodePA and main common electrodes CA are alternately arranged along thefirst direction X. The main pixel electrode PA and main commonelectrodes CA are arranged substantially parallel to each other. In thiscase, in the X-Y plane, neither of the main common electrodes CAoverlaps the main pixel electrode PA.

Specifically, one main pixel electrode PA is located between the maincommon electrode CAL and main common electrode CAR which neighbor eachother. In other words, the main common electrode CAL and main commonelectrode CAR are located on both sides of a position immediately abovethe main pixel electrode PA. Alternatively, the main pixel electrode PAis located between the main common electrode CAL and main commonelectrode CAR. Thus, the main common electrode CAL, main pixel electrodePA and main common electrode CAR are arranged in the named order alongthe first direction X.

The distance in the first direction X between the pixel electrode PE andcommon electrode CE is substantially uniform. Specifically, the distancein the first direction X between the main common electrode CAL and mainpixel electrode PA is substantially equal to the distance in the firstdirection X between the main common electrode CAR and main pixelelectrode PA.

In the embodiment, the distance in the first direction X between themain common electrode CA and main pixel electrode PA, that is, thedistance in the first direction X between mutually opposed edges of themain pixel electrode PA and main common electrode CA in the X-Y plane,is referred to as a horizontal inter-electrode distance HD. In theliquid crystal display of this embodiment, the horizontalinter-electrode distance HD is about 13 μm. If the horizontalinter-electrode distance HD is decreased, the voltage for controllingthe alignment state of a liquid crystal can be made relatively low, andlow power consumption is realized. Meanwhile, in some cases, since theaperture area in each pixel decreases, the brightness lowers. On theother hand, if the horizontal inter-electrode distance HD is increased,the aperture area in each pixel can be enlarged. However, a high voltageneeds to be applied in order to control the alignment state of theliquid crystal, and it is difficult to reduce power consumption. Thus,it is desirable to increase the horizontal inter-electrode distance HDas much as possible, so that the power consumption falls within apredetermined range. In the present embodiment, by adopting liquidcrystal materials which will be described later, the horizontalinter-electrode distance HD can be set in a range of 11 μm or more and13 μm or less.

FIG. 3 is a schematic cross-sectional view, taken along line A-A in FIG.2, showing a cross-sectional structure of the liquid crystal displaypanel LPN shown in FIG. 2. FIG. 3 shows only parts which are necessaryfor the description.

A backlight 4 is disposed on the back side of the array substrate ARwhich constitutes the liquid crystal display panel LPN. Various modesare applicable to the backlight 4. As the backlight 4, use may be madeof either a backlight which utilizes a light-emitting diode (LED) as alight source, or a backlight which utilizes a cold cathode fluorescentlamp (CCFL) as a light source. A description of the detailed structureof the backlight 4 is omitted.

The array substrate AR is formed by using a first insulative substrate10 having light transmissivity. Source lines S are formed on a firstinterlayer insulation film 11, and are covered with a second interlayerinsulation film 12. Gate lines (not shown) and storage capacitance lines(not shown) are disposed, for example, between the first insulativesubstrate 10 and first interlayer insulation film 11. Pixel electrodesPE are formed on the second interlayer insulation film 12. The pixelelectrode PE is located inside the positions immediately above theneighboring source lines S.

A first alignment film AL1 is disposed on that surface of the arraysubstrate AR, which is opposed to the counter-substrate CT, and thefirst alignment film AL1 extends over substantially the entirety of theactive area ACT. The first alignment film AL1 covers the pixelelectrodes PE, etc., and is also disposed over the second interlayerinsulation film 12. The first alignment film AL1 is formed of a materialwhich exhibits horizontal alignment properties.

In the meantime, the array substrate AR may also include a part of thecommon electrode CE.

The counter-substrate CT is formed by using a second insulativesubstrate 20 having light transmissivity. The counter-substrate CTincludes a black matrix BM, a color filter CF, an overcoat layer OC, acommon electrode CE, and a second alignment film AL2.

The black matrix BM partitions each pixel PX, and forms an apertureportion AP which is opposed to the pixel electrode PE. Specifically, theblack matrix BM is disposed so as to be opposed to wiring portions, suchas the source lines S, gate lines, storage capacitance lines, andswitching elements. In this example, only those portions of the blackmatrix BM, which extend in the second direction Y, are depicted, but theblack matrix BM may also include portions which extend in the firstdirection X. The black matrix BM is disposed on an inner surface 20A ofthe second insulative substrate 20, which is opposed to the arraysubstrate AR.

The color filter CF is disposed in association with each pixel PX.Specifically, the color filter CF is disposed in the aperture portion APon the inner surface 20A of the second insulative substrate 20, and apart of the color filter CF extends over the black matrix BM. Colorfilters CF, which are disposed in the pixels PX neighboring in the firstdirection X, have mutually different colors. For example, the colorfilters CF are formed of resin materials which are colored in threeprimary colors of red, blue and green. A red color filter CFR, which isformed of a resin material that is colored in red, is disposed inassociation with a red pixel. A blue color filter CFB, which is formedof a resin material that is colored in blue, is disposed in associationwith a blue pixel. A green color filter CFG, which is formed of a resinmaterial that is colored in green, is disposed in association with agreen pixel. Boundaries between these color filters CF are located atpositions overlapping the black matrix BM.

The overcoat layer OC covers the color filters CF. The overcoat layer OCreduces the effect of asperities on the surface of the color filters CF.

The common electrode CE is formed on that side of the overcoat layer OC,which is opposed to the array substrate AR. The distance in a thirddirection Z between the common electrode CE and pixel electrode PE issubstantially uniform. The third direction Z is a directionperpendicular to the first direction X and second direction Y, or anormal direction of the liquid crystal display panel LPN. In the presentembodiment, the distance in the third direction Z between the commonelectrode CE and pixel electrode PE, that is, the distance in the thirddirection Z between the mutually opposed surfaces of the pixel electrodePE and common electrode CE, is referred to as a vertical inter-electrodedistance VD. In the liquid crystal display of the present embodiment,the vertical inter-electrode distance VD is about 3 μm.

The second alignment film AL2 is disposed on that surface of thecounter-substrate CT, which is opposed to the array substrate AR, andthe second alignment film AL2 extends over substantially the entirety ofthe active area ACT. The second alignment film AL2 covers the commonelectrode CE and overcoat layer OC. The second alignment film AL2 isformed of a material which exhibits horizontal alignment properties.

The first alignment film AL1 and second alignment film AL2 are subjectedto alignment treatment (e.g. rubbing treatment or optical alignmenttreatment) for initially aligning the liquid crystal molecules of theliquid crystal layer LQ. A first alignment treatment direction PD1, inwhich the first alignment film AL1 initially aligns the liquid crystalmolecules, and a second alignment treatment direction PD2, in which thesecond alignment film AL2 initially aligns the liquid crystal molecules,are parallel to each other, and are opposite to or identical to eachother. For example, as shown in FIG. 2, the first alignment treatmentdirection PD1 and second alignment treatment direction PD2 aresubstantially parallel to the second direction Y, and are identical.

The above-described array substrate AR and counter-substrate CT aredisposed such that their first alignment film AL1 and second alignmentfilm AL2 are opposed to each other. In this case, columnar spacers,which are formed of, e.g. a resin material so as to be integral to oneof the array substrate AR and counter-substrate CT, are disposed betweenthe first alignment film AL1 of the array substrate AR and the secondalignment film AL2 of the counter-substrate CT. Thereby, a predeterminedcell gap, for example, a cell gap of 2 to 7 μm, is created. The arraysubstrate AR and counter-substrate CT are attached by a sealant SB onthe outside of the active area ACT in the state in which thepredetermined cell gap is created therebetween. In the meantime, thecell gap in this case is substantially equal to the above-describedvertical inter-electrode distance VD.

A first optical element OD1 is attached by, e.g. an adhesive, to anouter surface of the array substrate AR, that is, an outer surface 10Bof the first insulative substrate 10 which constitutes the arraysubstrate AR. The first optical element OD1 is located on that side ofthe liquid crystal display panel LPN, which is opposed to the backlight4, and controls the polarization state of incident light which entersthe liquid crystal display panel LPN from the backlight 4. The firstoptical element OD1 includes a first polarizer PL1 having a firstpolarization axis (or first absorption axis) AX1.

A second optical element OD2 is attached by, e.g. an adhesive, to anouter surface of the counter-substrate CT, that is, an outer surface 20Bof the second insulative substrate 20 which constitutes thecounter-substrate CT. The second optical element OD2 is located on thedisplay surface side of the liquid crystal display panel LPN, andcontrols the polarization state of emission light emerging from theliquid crystal display panel LPN. The second optical element OD2includes a second polarizer PL2 having a second polarization axis (orsecond absorption axis) AX2.

The first polarization axis AX1 of the first polarizer PL1 and thesecond polarization axis AX2 of the second polarizer PL2 have anorthogonal positional relationship (crossed Nicols). In this case, oneof the polarizers is disposed, for example, such that the polarizationaxis thereof is parallel or perpendicular to the initial alignmentdirection of liquid crystal molecules, that is, the first alignmenttreatment direction PD1 or second alignment treatment direction PD2.When the initial alignment direction is parallel to the second directionY, the polarization axis of one of the polarizers is parallel to thesecond direction Y or parallel to the first direction X.

In an example shown in part (a) of FIG. 2, the first polarizer PL1 isdisposed such that the first polarization axis AX1 thereof isperpendicular to the initial alignment direction (second direction Y) ofliquid crystal molecules LM (i.e. parallel to the first direction X),and the second polarizer PL2 is disposed such that the secondpolarization axis AX2 thereof is parallel to the initial alignmentdirection of liquid crystal molecules (i.e. parallel to the seconddirection Y).

In an example shown in part (b) of FIG. 2, the second polarizer PL2 isdisposed such that the second polarization axis AX2 thereof isperpendicular to the initial alignment direction (second direction Y) ofliquid crystal molecules LM (i.e. parallel to the first direction X),and the first polarizer PL1 is disposed such that the first polarizationaxis AX1 thereof is parallel to the initial alignment direction ofliquid crystal molecules (i.e. parallel to the second direction Y).

The liquid crystal layer LQ is held in the cell gap which is createdbetween the array substrate AR and the counter-substrate CT, and isdisposed between the first alignment film AL1 and second alignment filmAL2. The liquid crystal layer LQ is composed of, for example, a liquidcrystal material having a positive (positive-type) dielectric constantanisotropy.

In the above-described liquid crystal display, by the arrangement of thetwo polarizers PL1 and PL2, a black screen is displayed when no voltageis applied. In addition, by supplying a video signal to the pixelelectrode PE via the switching element SW from the source line S, ahorizontal electric field, which is substantially parallel to thesubstrate, is produced by a potential difference between the pixelelectrode PE and the common electrode CE. Liquid crystal moleculesrotate in a plane which is substantially parallel to the substrate majorsurface, so that the liquid crystal molecules may be aligned in thedirection of the horizontal electric field.

Next, the operation of the liquid crystal display panel LPN having theabove-described structure is described with reference to FIG. 2 and FIG.3.

Specifically, in a state in which no voltage is applied to the liquidcrystal layer LQ, that is, in a state (OFF time) in which no potentialdifference (or electric field) is produced between the pixel electrodePE and common electrode CE, the liquid crystal molecule LM of the liquidcrystal layer LQ is aligned such that the major axis thereof ispositioned in the first alignment treatment direction PD1 of the firstalignment film AL1 and the second alignment treatment direction PD2 ofthe second alignment film AL2. This OFF time corresponds to the initialalignment state, and the alignment direction of the liquid crystalmolecule LM at the OFF time corresponds to the initial alignmentdirection.

Strictly speaking, the liquid crystal molecule LM is not always alignedin parallel to the X-Y plane, and, in many cases, the liquid crystalmolecule LM is pre-tilted. Thus, the initial alignment direction of theliquid crystal molecule LM corresponds to a direction in which the majoraxis of the liquid crystal molecule LM at the OFF time is orthogonallyprojected onto the X-Y plane. In the description below, for the purposeof simplicity, it is assumed that the liquid crystal molecule LM isaligned in parallel to the X-Y plane, and the liquid crystal molecule LMrotates in a plane parallel to the X-Y plane.

In this case, each of the first alignment treatment direction PD1 andthe second alignment treatment direction PD2 is substantially parallelto the second direction Y. At the OFF time, the liquid crystal moleculeLM is initially aligned such that the major axis thereof issubstantially parallel to the second direction Y, as indicated by abroken line in FIG. 2. Specifically, the initial alignment direction ofthe liquid crystal molecule LM is parallel to the second direction Y (or0° to the second direction Y).

When the first alignment treatment direction PD1 and the secondalignment treatment direction PD2 are parallel and identical to eachother, as in the example illustrated, the liquid crystal molecules LMare substantially horizontally aligned (the pre-tilt angle issubstantially zero) in the middle part of the liquid crystal layer LQ inthe cross section of the liquid crystal layer LQ, and the liquid crystalmolecules LM are aligned with such pre-tilt angles that the liquidcrystal molecules LM become symmetric in the vicinity of the firstalignment film AL1 and in the vicinity of the second alignment film AL2,with respect to the middle part as the boundary (splay alignment).

As the result of the alignment treatment of the first alignment film AL1in the first alignment treatment direction PD1, the liquid crystalmolecules LM in the vicinity of the first alignment film AL1 areinitially aligned in the first alignment treatment direction PD1. As theresult of the alignment treatment of the second alignment film AL2 inthe second alignment treatment direction PD2, the liquid crystalmolecules LM in the vicinity of the second alignment film AL2 areinitially aligned in the second alignment treatment direction PD2. Whenthe first alignment treatment direction PD1 and the second alignmenttreatment direction PD2 are parallel and identical to each other, theliquid crystal molecules LM are splay-aligned, as described above, andthe alignment of the liquid crystal molecules LM in the vicinity of thefirst alignment film AL1 on the array substrate AR and the alignment ofthe liquid crystal molecules LM in the vicinity of the second alignmentfilm AL2 on the counter-substrate CT become symmetric in the up-and-downdirection with respect to the middle part of the liquid crystal layer LQas the boundary. Thus, optical compensation can be made even in adirection inclined to the normal direction of the substrate. Therefore,when the first alignment treatment direction PD1 and the secondalignment treatment direction PD2 are parallel and identical to eachother, light leakage is small in the case of black display, a highcontrast ratio can be realized, and the display quality can be improved.

In the meantime, when the first alignment treatment direction PD1 andthe second alignment treatment direction PD2 are parallel and oppositeto each other, the liquid crystal molecules LM are aligned withsubstantially equal pre-tilt angles, in the cross section of the liquidcrystal layer LQ, in the vicinity of the first alignment film AL1, inthe vicinity of the second alignment film AL2, and in the middle part ofthe liquid crystal layer LQ (homogeneous alignment).

Part of light from the backlight 4 passes through the first polarizerPL1 and enters the liquid crystal display panel LPN. The polarizationstate of the light, which enters the liquid crystal display panel LPN,varies depending on the alignment state of the liquid crystal moleculesLM at a time when the light passes through the liquid crystal layer LQ.At the OFF time, the light, which has passed through the liquid crystallayer LQ, is absorbed by the second polarizer PL2 (black display).

On the other hand, in a state in which a voltage is applied to theliquid crystal layer LQ, that is, in a state (ON time) in which apotential difference (or electric field) is produced between the pixelelectrode PE and common electrode CE, a lateral electric field (or anoblique electric field), which is substantially parallel to thesubstrates, is produced between the pixel electrode PE and the commonelectrode CE. The liquid crystal molecules LM are affected by theelectric field, and the major axes thereof rotate within a plane whichis parallel to the X-Y plane, as indicated by solid lines in the Figure.

In the example shown in FIG. 2, the liquid crystal molecule LM in aregion between the pixel electrode PE and main common electrode CALrotates clockwise relative to the second direction Y, and is aligned ina lower left direction in the Figure. The liquid crystal molecule LM ina region between the pixel electrode PE and main common electrode CARrotates counterclockwise relative to the second direction Y, and isaligned in a lower right direction in the Figure.

As has been described above, in the state in which the electric field isproduced between the pixel electrode PE and common electrode CE in eachpixel PX, the liquid crystal molecules LM are aligned in a plurality ofdirections, with boundaries at positions overlapping the pixel electrodePE, and domains are formed in the respective alignment directions.Specifically, a plurality of domains are formed in one pixel PX.

At such ON time, part of backlight, which is incident on the liquidcrystal display panel LPN from the backlight 4, passes through the firstpolarizer PL1, and enters the liquid crystal display panel LPN. Thelight entering the liquid crystal layer LQ varies the polarization statethereof. At the ON time, at least part of the light emerging from theliquid crystal layer LQ passes through the second polarizer PL2 (whitedisplay).

In the OFF state, the liquid crystal molecule LM is initially aligned ina direction which is substantially parallel to the second direction Y.In the ON state in which a potential difference is produced between thepixel electrode PE and common electrode CE, when the director of theliquid crystal molecule LM (or the major axis direction of the liquidcrystal molecule LM) is displaced by about 45° relative to the firstpolarization axis AX1 of the first polarizer PL1 and the secondpolarization axis AX2 of the second polarizer PL2 in the X-Y plane, anoptical modulation ratio of the liquid crystal becomes highest (i.e. thetransmittance at the aperture portion becomes maximum).

In the example illustrated, in the ON state, the director of the liquidcrystal molecule LM between the main common electrode CAL and pixelelectrode PE is substantially parallel to a 45° to 225° azimuthdirection in the X-Y plane, the director of the liquid crystal moleculeLM between the main common electrode CAR and pixel electrode PE issubstantially parallel to a 135° to 315° azimuth direction in the X-Yplane, and a peak transmittance is obtained. At this time, if attentionis paid to a transmittance distribution per pixel, the transmittance issubstantially zero on the pixel electrode PE and common electrode CE,and a high transmittance is obtained over substantially the entirety ofthe inter-electrode gap between the pixel electrode PE and commonelectrode CE.

In the meantime, the main common electrode CAL, which is locatedimmediately above the source line S1, and the main common electrode CAR,which is located immediately above the source line S2, are opposed tothe black matrices BM. Each of the main common electrode CAL and maincommon electrode CAR has a width which is equal to or less than thewidth in the first direction X of the black matrix BM, and does notextend to the pixel electrode PE side from the position overlapping theblack matrix BM. Thus, in each pixel, the aperture portion whichcontributes to display corresponds to areas between the pixel electrodePE, on the one hand, and the main common electrode CAL and main commonelectrode CAR, on the other hand, of the area between the black matricesBM or the area between the source line S1 and source line S2.

According to the present embodiment, therefore, a decrease intransmittance can be suppressed. Thereby, degradation in display qualitycan be suppressed.

In the liquid crystal display of this embodiment, there was a case inwhich the electric field could not effectively be used depending onconditions such as a dielectric constant anisotropy Δε of a liquidcrystal material and a horizontal inter-electrode distance HD, and thetransmittance lowered. For example, when the same liquid crystalmaterial as in a liquid crystal display of an IPS mode or an FFS mode,which makes use of a lateral electric field, was used in theabove-described liquid crystal display, a sufficiently brightness couldnot be obtained.

Taking the above into account, the inventors found the conditions ofliquid crystal materials, which are necessary for obtaining a sufficientbrightness in the above-described liquid crystal display. Using acomparative example, a description will be given below of liquid crystalmaterials which are applied to the liquid crystal display of the presentembodiment.

FIG. 4 shows an example of the relationship between a refractive indexanisotropy Δn and a dielectric constant anisotropy Δε, with respect toliquid crystal materials which are applied to the liquid crystal displayof the embodiment and liquid crystal materials which are applied to aliquid crystal display of a comparative example.

FIG. 4 shows a scatter diagram in which a plurality of liquid crystalmaterials are plotted, with the abscissa indicating the refractive indexanisotropy Δn and the ordinate indicating the dielectric constantanisotropy Δε, and graphs G1 and G2 of approximate expressions.Diamond-shaped plots indicate liquid crystal materials which wereapplied to an IPS mode or FFS mode liquid crystal display of thecomparative example, and each diamond-shaped plot indicates a liquidcrystal material in the case where a sufficient brightness was obtained.A square-shaped plot P1 indicates a liquid crystal material with which asufficient brightness was obtained when this liquid crystal material wasapplied to the liquid crystal display of the embodiment. The liquidcrystal material of the plot P1 has a dielectric constant anisotropyΔεof 16, and a refractive index anisotropy Δn of about 0.1.

In the liquid crystal display of the comparative example, both the pixelelectrode PE and common electrode CE are disposed on the array substrateAR and the alignment state of the liquid crystal is controlled by makinguse of a lateral electric field which is produced between the pixelelectrode PE and common electrode CE (e.g. a liquid crystal display ofan FFS mode or an IPS mode). In the liquid crystal display of thecomparative example, the inter-electrode distance (horizontalinter-electrode distance) in the first direction X between the pixelelectrode PE and common electrode CE is in a range of 3 μm or more and 4μm or less.

Besides, in each of the liquid crystal display of the embodiment and theliquid crystal display of the comparative example, the gap (verticalinter-electrode distance) between the array substrate AR andcounter-substrate CT is about 3 μm, and the voltage applied to theliquid crystal layer LQ at the time of white display is about 5 V.

The approximate graph G1 shown in FIG. 4 is a graph extending throughthe plots of the liquid crystal materials of the liquid crystal displayof the embodiment and has the same inclination as the approximate graphG2. Incidentally, the approximate expression of the plots of liquidcrystal materials of the liquid crystal display of the comparativeexample was calculated, for example, by a least square method.

When the liquid crystal material (dielectric constant anisotropy Δε=16)of the plot P1 was applied to the liquid crystal display of theembodiment, a brightness, which is 90% to 95% of the brightness(transmittance) of the liquid crystal display of the comparativeexample, was obtained.

In addition, by simulation, when a liquid crystal material having adielectric constant anisotropy Δε of 14 and a refractive indexanisotropy Δn of about 0.1 was applied to the liquid crystal display ofthe embodiment, a brightness, which is 80% or more of the brightness ofthe liquid crystal display of the comparative example, was obtained.

When a liquid crystal material having a dielectric constant anisotropyΔε of 10 and a refractive index anisotropy Δn of about 0.1 was appliedto the liquid crystal display of the embodiment, a brightness, which is70% or more of the brightness of the liquid crystal display of thecomparative example, was obtained. In the case of the brightness whichis 70% or more of the brightness of the liquid crystal display of thecomparative example, a brightness which is equal to the brightness ofthe liquid crystal display of the comparative example was successfullyobtained without lowering display quality, by increasing the liquidcrystal driving voltage at the time of white display or by increasingthe luminance of the backlight.

From the above-described results, in the liquid crystal display of thepresent embodiment, the liquid crystal material should preferably havethe dielectric constant anisotropy Δε of 10 or more, and should morepreferably have the dielectric constant anisotropy Δε of 14 or more.Furthermore, if the dielectric constant anisotropy As of the liquidcrystal material is set at about 16, a liquid crystal display having asufficient brightness can be obtained without increasing the liquidcrystal driving voltage at the time of white display or increasing theluminance of the backlight.

Besides, according to the embodiment, a high transmittance is obtainedin the inter-electrode gap between the pixel electrode PE and commonelectrode CE. Thus, a sufficiently high transmittance per pixel can beobtained by increasing the inter-electrode distance between the pixelelectrode PE, on the one hand, and the main common electrode CAL andmain common electrode CAR, on the other hand. In the present embodiment,the liquid crystal display with a sufficient brightness was successfullyobtained by using the above-described liquid crystal materials, bysetting the horizontal inter-electrode distance HD in a range of 11 μmor more and 13 μm or less. In the meantime, the horizontalinter-electrode distance HD is greater than the vertical inter-electrodedistance VD, or in other words, the horizontal inter-electrode distanceHD is greater than the thickness of the liquid crystal layer.

In addition, if the liquid crystal driving voltage at the time of whitedisplay is increased or the luminance of the backlight is increased,power consumption would increase. However, by selecting a proper liquidcrystal material, it is possible to avoid an increase in powerconsumption.

Specifically, according to the present embodiment, it is possible toprovide a liquid crystal display which can suppress, with use of aproper liquid crystal material, a decrease in brightness, and cansuppress degradation in display quality.

As regards product specifications in which the pixel pitch is different,the peak condition of the transmittance distribution can be used byvarying the inter-electrode distance (i.e. by varying the position ofdisposition of the main common electrodes CA relative to the pixelelectrode PE that is disposed at substantially the center of the pixelPX). Specifically, in the display mode of the present embodiment,products with various pixel pitches can be provided by setting theinter-electrode distance, without necessarily requiring fine electrodeprocessing, as regards the product specifications from low-resolutionproduct specifications with a relatively large pixel pitch tohigh-resolution product specifications with a relatively small pixelpitch. Therefore, requirements for high transmittance and highresolution can easily be realized.

According to the present embodiment, if attention is paid to thetransmittance distribution in the region overlapping the black matrixBM, the transmittance is sufficiently lowered in this region. The reasonfor this is that the electric field does not leak to the outside of thepixel from the position of the common electrode CE, and an undesiredlateral electric field does not occur between pixels which neighbor eachother with the black matrix BM interposed, and therefore the liquidcrystal molecules in the region overlapping the black matrix BM keep theinitial alignment state, like the case of the OFF time (or black displaytime). Accordingly, even when the colors of the color filters aredifferent between neighboring pixels, the occurrence of color mixturecan be suppressed, and the decrease in color reproducibility or thedecrease in contrast ratio can be suppressed.

When misalignment occurs between the array substrate AR and thecounter-substrate CT, there are cases in which a difference occurs inthe horizontal inter-electrode distance HD between the pixel electrodePE and the common electrodes CE on both sides of the pixel electrode PE.However, since such misalignment commonly occurs in all pixels PX, theelectric field distribution does not differ between the pixels PX, andthe influence on the display of images is very small. In addition, evenwhen misalignment occurs between the array substrate AR and thecounter-substrate CT, leakage of an undesired electric field to theneighboring pixel can be suppressed. Thus, even when the colors of thecolor filters differ between neighboring pixels, the occurrence of colormixture can be suppressed, and the decrease in color reproducibility orthe decrease in contrast ratio can be suppressed.

According to the present embodiment, the main common electrodes CA areopposed to the source lines S. In particular, in the case where the maincommon electrode CAL and main common electrode CAR are disposedimmediately above the source line S1 and source line S2, compared to thecase where the main common electrode CAL and main common electrode CARare disposed on the pixel electrode PE side of the source line S1 andsource line S2, the aperture portion AP can be enlarged and thetransmittance of the pixel PX can be enhanced.

In addition, by disposing the main common electrode CAL and main commonelectrode CAR immediately above the source line S1 and source line S2,the inter-electrode distance between the pixel electrode PE, on the onehand, and the main common electrode CAL and main common electrode CAR,on the other hand, can be increased, and a lateral electric field, whichis closer to a horizontal lateral electric field, can be produced.Therefore, a wide viewing angle, which is the advantage of an IPS mode,etc. in the conventional structure, can be maintained.

According to the present embodiment, a plurality of domains can beformed in one pixel. Thus, the viewing angle can optically becompensated in plural directions, and a wide viewing angle can berealized.

The above-described example is directed to the case where the initialalignment direction of liquid crystal molecules LM is parallel to thesecond direction Y. However, the initial alignment direction of liquidcrystal molecules LM may be an oblique direction D which obliquelycrosses the second direction Y, as shown in FIG. 2. An angle θ1 formedbetween the second direction Y and the initial alignment direction D is0° or more and 45° or less. From the standpoint of alignment control ofliquid crystal molecules LM, it is very effective that the angle θ1 isabout 5° to 30°, more preferably 20° or less. Specifically, it isdesirable that the initial alignment direction of liquid crystalmolecules LM be substantially parallel to a direction in a range of 0°to 20°, relative to the second direction Y.

The above-described example relates to the case in which the liquidcrystal layer LQ is composed of a liquid crystal material having apositive (positive-type) dielectric constant anisotropy. Alternatively,the liquid crystal layer LQ may be composed of a liquid crystal materialhaving a negative (negative-type) dielectric constant anisotropy.Although a detailed description is omitted, in the case of thenegative-type liquid crystal material, since the positive/negative stateof dielectric constant anisotropy is reversed, it is desirable that theabove-described formed angle θ1 be within the range of 45° to 90°,preferably the range of 70° or more.

Since a lateral electric field is hardly produced over the pixelelectrode PE or common electrode CE even at the ON time (or an electricfield enough to drive liquid crystal molecules LM is not produced), theliquid crystal molecules LM scarcely move from the initial alignmentdirection, like the case of the OFF time. Thus, even if the pixelelectrode PE and common electrode CE are formed of a light-transmissive,electrically conductive material such as ITO, little backlight passesthrough these regions, and these regions hardly contribute to display atthe ON time. Thus, the pixel electrode PE and common electrode CE inthis embodiment do not necessarily need to be formed of a transparent,electrically conductive material, and may be formed of an electricallyconductive material such as aluminum, silver, or copper.

In the present embodiment, the structure of the pixel PX is not limitedto the example shown in FIG. 2. For example, a plurality of main pixelelectrodes PA may be disposed in one pixel PX, and a plurality of maincommon electrodes CA may be disposed on both sides of each of the mainpixel electrodes. In this case, too, the same advantageous effects as inthe above-described embodiment can be obtained by setting the horizontalinter-electrode distance HD in a range of 11 μm or more and 13 μm orless and by selecting a proper liquid crystal material.

In the present embodiment, the common electrode CE may include, inaddition to the main common electrodes CA provided on thecounter-substrate CT, second main common electrodes which are providedon the array substrate AR, are opposed to the main common electrodes CA(or opposed to the source lines S) and are electrically isolated fromthe pixel electrode PE. The second main common electrodes extendsubstantially in parallel to the main common electrodes CA and have thesame potential as the main common electrodes CA. By providing suchsecond main common electrodes, an undesired electric field from thesource lines S can be shielded.

In addition, the common electrode CE may include, in addition to themain common electrodes CA provided on the counter-substrate CT, secondsub-common electrodes which are provided on the array substrate AR andare opposed to the gate line G and storage capacitance lines C. Thesecond sub-common electrodes extend in a direction crossing the maincommon electrodes CA and have the same potential as the main commonelectrodes CA. By providing such second sub-common electrodes, anundesired electric field from the gate line G and storage capacitancelines C can be shielded. According to the structure including suchsecond main common electrodes and second sub-common electrodes,degradation in display quality can further be suppressed.

As has been described above, according to the present embodiment, it ispossible to provide a liquid crystal display which can suppressdegradation in display quality.

In the meantime, in the above-described embodiment, the liquid crystaldisplay with good display quality is provided by properly setting theinter-electrode distance in the first direction X and the dielectricconstant anisotropy of the liquid crystal material. The reason why thevertical inter-electrode distance VD is not specified is that if thevertical inter-electrode distance VD varies, the distance between themain pixel electrode PA and main common electrode CA also varies andaffects the display quality, but the effect of the variation of thevertical inter-electrode distance VD is less than the effect of thevariation of the horizontal inter-electrode distance HD. Incidentally,the vertical inter-electrode distance VD should preferably be in a rangeof 2 μm or more and 4 μm or less. In this case, it is possible toprovide a liquid crystal display with good display quality and reducedpower consumption.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A liquid crystal display comprising: a firstsubstrate including a main pixel electrode; a second substrate includinga main common electrode extending substantially in parallel to the mainpixel electrode on both sides of the main pixel electrode; and a liquidcrystal layer including liquid crystal molecules held between the firstsubstrate and the second substrate, wherein a horizontal inter-electrodedistance in a first direction between the main pixel electrode and themain common electrode is in a range of 11 μm or more and 13 μm or less,and a dielectric constant anisotropy of the liquid crystal layer is 10or more.
 2. The liquid crystal display of claim 1, wherein thedielectric constant anisotropy of the liquid crystal layer is 14 ormore.
 3. The liquid crystal display of claim 1, wherein the dielectricconstant anisotropy of the liquid crystal layer is
 16. 4. The liquidcrystal display of claim 1, wherein a vertical inter-electrode distancebetween the main pixel electrode and the main common electrode is about3 μm.
 5. The liquid crystal display of claim 2, wherein a verticalinter-electrode distance between the main pixel electrode and the maincommon electrode is about 3 μm.
 6. The liquid crystal display of claim3, wherein a vertical inter-electrode distance between the main pixelelectrode and the main common electrode is about 3 μm.
 7. The liquidcrystal display of claim 1, wherein a maximum voltage, which is appliedto the liquid crystal layer, is 5 V or less.
 8. The liquid crystaldisplay of claim 2, wherein a maximum voltage, which is applied to theliquid crystal layer, is 5 V or less.
 9. The liquid crystal display ofclaim 3, wherein a maximum voltage, which is applied to the liquidcrystal layer, is 5 V or less.
 10. The liquid crystal display of claim4, wherein a maximum voltage, which is applied to the liquid crystallayer, is 5 V or less.
 11. The liquid crystal display of claim 5,wherein a maximum voltage, which is applied to the liquid crystal layer,is 5 V or less.
 12. The liquid crystal display of claim 6, wherein amaximum voltage, which is applied to the liquid crystal layer, is 5 V orless.
 13. A liquid crystal display comprising: a first substrateincluding source lines extending in a second direction, a gate lineextending in a first direction which is substantially perpendicular tothe second direction, a storage capacitance line extending in the firstdirection, a main pixel electrode extending in the second directionbetween the source lines, a contact portion which is electricallyconnected to the main pixel electrode and is disposed in an upper layerof the storage capacitance line, and a switching element configured toswitch a connection between the source line and the main pixel electrodeby a signal which is applied to the gate line; a second substrateincluding a main common electrode extending substantially in parallel tothe main pixel electrode on both sides of the main pixel electrode; anda liquid crystal layer including liquid crystal molecules held betweenthe first substrate and the second substrate, wherein when a horizontalinter-electrode distance in the first direction between the main pixelelectrode and the main common electrode is in a range of 11 μm or moreand 13 μm or less, a dielectric constant anisotropy of the liquidcrystal layer is 10 or more.
 14. A liquid crystal display comprising: afirst substrate including source lines, a gate line crossing the sourcelines, a main pixel electrode extending along the source lines betweenthe source lines, and a switching element configured to switch aconnection between the source line and the main pixel electrode by asignal which is applied to the gate line; a second substrate including amain common electrode extending substantially in parallel to the mainpixel electrode on both sides of the main pixel electrode; and a liquidcrystal layer including liquid crystal molecules held between the firstsubstrate and the second substrate, wherein a horizontal inter-electrodedistance between the main pixel electrode and the main common electrodeis greater than a thickness of the liquid crystal layer, and adielectric constant anisotropy of the liquid crystal layer is 10 ormore.
 15. The liquid crystal display of claim 14, wherein the horizontalinter-electrode distance is in a range of 11 μm or more and 13 μm orless.
 16. The liquid crystal display of claim 15, wherein the thicknessof the liquid crystal layer is in a range of 2 μm or more and 4 μm orless.
 17. The liquid crystal display of claim 16, wherein the dielectricconstant anisotropy of the liquid crystal layer is 14 or more.
 18. Theliquid crystal display of claim 17, wherein the dielectric constantanisotropy of the liquid crystal layer is
 16. 19. The liquid crystaldisplay of claim 17, wherein a vertical inter-electrode distance betweenthe main pixel electrode and the main common electrode is about 3 μm.20. The liquid crystal display of claim 17, wherein a maximum voltage,which is applied to the liquid crystal layer, is 5 V or less.